The present invention relates to iron oxide particles. More particularly, the present invention relates to iron oxide particles which are capable of suppressing to the minimum the deterioration of a resin, for example, a resin containing chlorine in the molecular and colored with iron oxide particles, which is caused by the influence of the heat applied to the resin during processing or the light during exposure to the open air, and the gelation of such a resin which is caused during the process of producing a coating material; to iron oxide particles as magnetic particles for a magnetic toner which have an excellent fluidity; and to a method of producing such iron oxide particles.
With the rise of the level of recent life, commercial goods have been required to have not only improved functional qualities but also a good external appearance which suits customers' tastes and senses of beauty. With respect to colors, various colors are demanded. In order to satisfy such demand, for example, a coloring pigment is mixed and kneaded with a resin when resin moldings are produced, and a coloring pigment is mixed and dispersed in a vehicle when a coating material is produced.
However, it is widely known that the resin in molding articles produced from the resin and a coloring pigment mixed and kneaded therewith is deteriorated by the influence of the heat applied to the resin during processing or the light during exposure to the open air, and that the coating material produced from a vehicle and a coloring pigment dispersed therein gelates by the influence of the coloring pigment.
The typical coloring pigments which have been generally used are iron oxide particles. Hematite iron oxide particles are used as a reddish brown pigment, maghemite iron oxide particles are used as a brown pigment, and magnetite iron oxide particles are used as a black pigment. These iron oxide particles among all the pigments are especially known to accelerate the deterioration and the gelation of a resin. This fact is clear from the description at p. 56 of The Journal of the Japan Society of Color Material, vol 3, published by the Japan Society of Color Material: "The influence of a pigment on the thermal decomposition of commercially available styrene resin, methacryl resin and styrene-acrylonitrile resin during the molding process was investigated. As a result of further investigation of the cause of thermal decomposition, it was concluded that the thermal decomposition of each resin was caused by some metals contained in a pigment. It was found that pigments which contain iron, cadmium and the like (i.e., colcothar(hematite), cadmium red, cadmium yellow, etc.) comparatively accelerates the thermal decomposition of each of the these resins.
The deterioration of a resin containing chlorine will be explained below, citing a polyvinyl chloride resin which is typical of chlorine-containing resins, as an example.
Polyvinyl chloride resins, even if they are not colored with iron oxide particles, are decomposed and degenerated by the heat applied thereto during processing or the sun's light during exposure to the open air after they are molded into articles.
Such deterioration is produced because a part of chlorine bonding in a polyvinyl chloride resin is decomposed at 100.degree. to 200.degree. C. by the action of heat and generates hydrogen chloride, thereby forming a polyene structure having a double bonding, and further the hydrogen chloride generated secondarily acts on the resin itself, thereby cutting a part of C--C bonding in a polymer or producing a cross-linking as a chain reaction.
It is generally known that in order to suppress such deterioration, a neutralizer such as a lead compound and metallic soap for neutralizing the hydrogen chloride generated by decomposition or a resin stabilizer such as an organic tin compound and an epoxy compound which is effective in suppressing the generation of a double bonding is added to a polyvinyl chloride resin for an ordinary thermoforming material even when it is not colored by an iron oxide pigment.
As is known, since iron oxide particles accelerate the dehydrochlorination of a resin by the influence of Fe which is a component of the iron oxide particles, in the case of mixing as a coloring agent iron oxide particles with a polyvinyl chloride resin, the deterioration of the resin is greatly accelerated.
Iron oxide particles which are capable of suppressing the deterioration or the gelation of a resin to the minimum is therefore strongly demanded.
In order to suppress the deterioration or the gelation of a resin, a method of coating the surfaces of iron oxide particles with various inorganic or organic compounds so as to suppress the surface activity of the iron oxide particles is conventionally adopted. As a method of coating the surfaces of iron oxide particles with an inorganic compound, a method of forming a continuous silica film on the surfaces of iron oxide particles (Japanese Patent Publication (KOKOKU) No. 54-7292 (1979)), a method of coating the surfaces of the iron oxide particles with glassy Na and/or K-Si (Japanese Patent Publication No. 53-11537 (1978)), a method of depositing SiO.sub.2 on the surfaces of the iron oxide particles and further depositing aluminum hydroxide on the precipitated SiO.sub.2 (Japanese Patent Application Laid-Open (KOKAI) No. 53-36538 (1978)), etc. are known. As a method of coating the surfaces of iron oxide particles with an organic compound, there is, for example, a method of kneading an organic compound having a hydrophobic group with the iron oxide particles by a wheel-type kneader or an attrition mill (Japanese Patent Application Laid-Open (KOKAI) No. 3-221965 (1991)).
Although iron oxide particles which are capable of suppressing the deterioration or the gelation of a resin to the minimum is now in the strongest demand, the above-described known iron oxide particles cannot be said to suppress the deterioration or the gelation of a resin sufficiently, because it is difficult to coat the surfaces of the iron oxide particles with inorganic fine particles uniformly, as will be shown in a later-described comparative examples.
On the other hand, a development process using, as a developer, composite particles which are produced by mixing and dispersing magnetic particles such as magnetite particles with a resin without using a carrier, in other words, what is called a one-component magnetic toner is well known and generally used as one of the electrostatic latent image development processes.
With the recent development of a high-speed copying machine, it is strongly demanded to enhance the fine line reproducibility, to improve the image quality such as the density of an image and the gradient. For this purpose, a magnetic toner which has an improved charging stability, an improved blackness or a large residual magnetization and an excellent fluidity has been demanded.
As to the fluidity of a magnetic toner, Japanese Patent Application Laid-Open (KOKAI) No. 53-94932 (1991) describes, "Such a high-resistance magnetic toner has too low fluidity for even development due to its high resistance. That is, although a high-resistance toner for PPC can hold the necessary charge for transfer, since there is a slight amount of charge in the toner bottle, on the surface of a magnetic roll, etc. in the process other than the transfer process which needs no charge, due to the frictional charge or the mechanoerectret in the process of producing the toner, agglomeration is apt to be caused, which lowers the fluidity . . . ", and "Another object of the present invention is to provide a high-resistance toner for PPC having improved fluidity so as to produce a high-quality indirect photocopy free from uneven development and, hence, having excellent definition and gradient".
In order to obtain a magnetic toner having the above-described properties, it is necessary to satisfy one of the following requirements in accordance with the developing system:
(1) The residual magnetization of the magnetic particles is as low as possible so that the agglomerating force is small and the fluidity is excellent.
(2) The Fe.sup.2+ content of the magnetic particles is large so that the blackness is high, in other words, the magnetic particles show a bluish black color and are excellent in the fluidity.
(3) The magnetic particles have a large residual magnetization and high fluidity.
In the case (1), a good toner is obtained as described in Japanese Patent Application Laid-Open (KOKAI) No. 3-122658 (1992), "Such a magnetic material has a small residual magnetization so that the agglomeration force is also small, which facilitates the production of a good image." As magnetic particles having a small residual magnetization, spherical spinel-type iron oxide particles have conventionally known. Due to the small residual magnetization, the agglomerating force is very small. The residual magnetization of magnetic particles has a close relationship with the particle size thereof. The smaller the particle size of the magnetic particles is, the larger the residual magnetization is apt to be. The relative value of residual magnetization of the spherical magnetic particles (the area-average particle diameter).times.(the measured residual magnetization)! is not more than 2 (.mu.m.multidot.emu/g).
In the case (2), the blackness of the particles having a particle diameter of about 0.1 to 0.5 .mu.m which are used for a magnetic toner is known to be dependent mainly on the Fe.sup.2+ content. As the Fe.sup.2+ content becomes large, the particles show a bluish black color, as described in Journal of the Japan Society of Powder and Powder Metallurgy, Vol. 26 (1979) page 240, "The blackness of the sample is dependent on the Fe(II) content and the average particle diameter, and particles having an average particle diameter of 0.2 .mu.m show a bluish black color, and are the most suitable as a black pigment. . . . If the Fe(II) content is not less than 10 wt %, any of the samples has a black color although there is a slight difference in the blackness. When the Fe(II) content is reduced to less than 10 wt %, the color of each sample changes from black to reddish brown." As the magnetic particles having a high blackness, hexahedral spinel-type iron oxide particles are conventionally known. These particles have a larger Fe.sup.2+ content of 0.3 to 0.5 in molar ratio based on the Fe.sup.3+ content and, as a result, the blackness is high.
In the case of (3), as the magnetic particles having a high residual magnetization, octahedral spinel-type iron oxide particles are conventionally known. The residual magnetization of magnetic particles has a close relationship with the particle size thereof. The smaller the particle size of the magnetic particles is, the larger the residual magnetization is apt to be. The relative value of residual magnetization of the octahedral magnetic particles (the area-average particle diameter).times.(the measured residual magnetization)! is more than 2 (.mu.m.multidot.emu/g).
In order to enhance the fluidity of a magnetic toner, attempts have been mainly made to subject the magnetic toner itself to some treatment. For example, (i) a method of kneading the fine particles of a silicon compound or the like, which is a fluidity modifier, with a resin, so that the fluidity modifier is contained in the surface or within the magnetic toner (Japanese Patent Application Laid-Open (KOKAI) Nos. 53-94932 (1978), 59-223451 (1984) , 60-26953 (1985) and 2-73362 (1990), and Japanese Patent Publication No. 4-21860 (1992), and (ii) a method of treating the surfaces of the particles of a magnetic toner with fine particles of a silicon compound or the like, which is a fluidity modifier (Japanese Patent Application Laid-Open (KOKAI) No. 63-8750 (1985)) are known.
On the other hand, as a method of treating the magnetic particles of a magnetic toner with a silicon compound, so that the silicon compound is present on the surfaces of the particles, for example, (iii) a method of adhering and bonding silica gel SiO.sub.2 --nH.sub.2 O to the surfaces of the magnetic particles (Japanese Patent Application Laid-Open (KOKAI) No. 2-73362 (1990)), and (iv) a method of forming on the surfaces of the magnetic particles a neutralized silicate compound film (Japanese Patent Application Laid-Open (KOKAI) No. 57-201244 (1982)) are known.
However, the method (i) of containing in a magnetic toner a fluidity modifier is defective in that the fluidity modifier scales off and the fluidity thereof reduces with the passage of time, when magnetic toner particles comes into contact with another magnetic toner particles or with the sleeve, thereby lowering the fluidity with the passage of time.
In the method (ii), fine particles of the silicon compound as the fluidity modifier scales off the surface of the magnetic toner particles by the shock of the contact between the magnetic toner particles or between the magnetic toner particles and the sleeve when the magnetic toner is charged on the sleeve.
The object of the method (iii) is to improve of the environmental stability by suppressing the change in the amount of charge due to the change in the humidity by utilizing the water absorbing property and the water releasing property of the silica gel SiO.sub.2 --nH.sub.2 O which is adhered and bonded to the surfaces of the magnetic particles so as to maintain the amount of charge at a constant value, but the improvement of the fluidity of the magnetic toner is insufficient, as shown in a later-described comparative example.
In the method (iv), since the neutralized silicate compound exists on the surfaces of the magnetic particles not in the form of fine particles but in the form of a film, the adhesion between the magnetic toner increases in comparison with that in which fine particles exist between the magnetic toner, so that it is impossible to improve a fluidity of the magnetic toner.
As a result of studies undertaken by the present inventors in order to eliminated the above-mentioned defects, it has been found that by adding or precipitating 0.25 to 10 parts by weight (calculated as an oxide) of the fine particles of the oxygen compound and/or the fine particles of the hydrate of the element selected from the group consisting of Al, Si, Zr and Ti to or onto 100 parts by weight of iron oxide particles having a specific surface area represented by the formula: 6/(.rho..multidot.d.sub.3).times..phi..sub.s .rho. represents a specific gravity of the iron oxide particles, d.sub.3 represents an area-average particle diameter (0.1 .mu.m.ltoreq.d.sub.3 .ltoreq.1.0 .mu.m), and .phi..sub.s represents an area shape factor (1.0.ltoreq..phi..sub.s .ltoreq.2.0)!, and mixing the iron oxide particles and the fine particles by compressing, shearing and spatula-stroking by in a wheel-type kneader, the obtained iron oxide particles with the fine particles of an oxygen compound and/or the fine particles of a hydrate of one element selected from the group consisting of Al, Si, Zr and Ti adhered to (deposited on) the surfaces thereof are capable of suppressing the deterioration or the gelation of a resin to the minimum, and that in case of using spherical, hexahedral or octahedral spinel-type iron oxide particles as the iron oxide particle to be treated, the obtained iron oxide particles with fine silica particles adhered to (deposited on) the surface thereof have an excellent fluidity and are useful as magnetic particles for a magnetic toner. The present invention has been achieved on the basis of these findings.